CY28442
Byte 10: Control Register 10 (continued)
Bit
@Pup
Name
CLKREQ#A
Description
SRC[T/C]5 CLKREQ#A control
2
1
0
0
1 = SRC[T/C]5 stoppable by CLKREQ#A pin
0 = SRC[T/C]5 not controlled by CLKREQ#A pin
0
0
CLKREQ#A
CLKREQ#A
SRC[T/C]3 CLKREQ#A control
1 = SRC[T/C]3 stoppable by CLKREQ#A pin
0 = SRC[T/C]3 not controlled by CLKREQ#A pin
SRC[T/C]1 CLKREQ#A control
1 = SRC[T/C]1 stoppable by CLKREQ#A pin
0 = SRC[T/C]1 not controlled by CLKREQ#A pin
The CY28442 requires a Parallel Resonance Crystal. Substi-
tuting a series resonance crystal will cause the CY28442 to
operate at the wrong frequency and violate the ppm specifi-
cation. For most applications there is a 300-ppm frequency
shift between series and parallel crystals due to incorrect
loading.
Table 5. Crystal Recommendations
Frequency
(Fund)
Drive
(max.)
Shunt Cap Motional
(max.)
Tolerance
(max.)
Stability
(max.)
Aging
(max.)
Cut
Loading Load Cap
Parallel 20 pF
(max.)
14.31818 MHz
AT
0.1 mW
5 pF
0.016 pF
35 ppm
30 ppm
5 ppm
The following diagram shows a typical crystal configuration
using the two trim capacitors. An important clarification for the
following discussion is that the trim capacitors are in series
with the crystal not parallel. It’s a common misconception that
load capacitors are in parallel with the crystal and should be
approximately equal to the load capacitance of the crystal.
This is not true.
Crystal Loading
Crystal loading plays a critical role in achieving low ppm perfor-
mance. To realize low ppm performance, the total capacitance
the crystal will see must be considered to calculate the appro-
priate capacitive loading (CL).
Figure 1. Crystal Capacitive Clarification
crystal. This means the total capacitance on each side of the
crystal must be twice the specified crystal load capacitance
(CL). While the capacitance on each side of the crystal is in
series with the crystal, trim capacitors (Ce1,Ce2) should be
calculated to provide equal capacitive loading on both sides.
Calculating Load Capacitors
In addition to the standard external trim capacitors, trace
capacitance and pin capacitance must also be considered to
correctly calculate crystal loading. As mentioned previously,
the capacitance on each side of the crystal is in series with the
Rev 1.0,November 21, 2006
Page 9 of 21
SPECTRALINEAR [ SPECTRALINEAR INC ]
